Surface acoustic wave (saw) resonator

ABSTRACT

A surface acoustic wave (SAW) resonator includes a piezoelectric layer disposed over a substrate, and a plurality of electrodes disposed over the first surface of the piezoelectric layer. A layer is disposed between the substrate and the piezoelectric layer. A surface of the layer has a smoothness sufficient to foster atomic bonding between layer and the piezoelectric layer. A plurality of features provided on a surface of the piezoelectric layer reflects acoustic waves and reduces the incidence of spurious modes in the piezoelectric layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part under 37 C.F.R.§1.53(b) of, and claims priority under 35 U.S.C. §120 from, U.S. patentapplication Ser. No. 14/835,679 filed on Aug. 25, 2015, naming StephenRoy Gilbert, et al. as inventors. The entire disclosure of U.S. patentapplication Ser. No. 14/835,679 is specifically incorporated herein byreference.

BACKGROUND

Electrical resonators are widely incorporated in modern electronicdevices. For example, in wireless communications devices, radiofrequency (RF) and microwave frequency resonators are used in filters,such as filters having electrically connected series and shuntresonators forming ladder and lattice structures. The filters may beincluded in a duplexer (diplexer, triplexer, quadplexer, quintplexer,etc.) for example, connected between an antenna (there could be severalantennas like for MIMO) and a transceiver for filtering received andtransmitted signals.

Various types of filters use mechanical resonators, such as surfaceacoustic wave (SAW) resonators. The resonators convert electricalsignals to mechanical signals or vibrations, and/or mechanical signalsor vibrations to electrical signals.

While certain surface modes are desired, certain standing spurious modescan exist between the opposing faces of the piezoelectric material ofthe SAW resonator. These spurious modes are parasitic, and can impactthe performance of filters comprising SAW resonators.

What is needed, therefore, is a SAW resonator structure that overcomesat least the shortcomings of known SAW resonators described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The example embodiments are best understood from the following detaileddescription when read with the accompanying drawing figures. It isemphasized that the various features are not necessarily drawn to scale.In fact, the dimensions may be arbitrarily increased or decreased forclarity of discussion. Wherever applicable and practical, like referencenumerals, refer to like elements.

FIG. 1A is a top view of a SAW resonator structure according to arepresentative embodiment.

FIG. 1B is a graph of admittance versus frequency.

FIG. 1C is the cross-sectional view of a SAW resonator structure of FIG.1A along line 1C-1C.

FIG. 1D is a cross-sectional view of a portion of the SAW resonatorstructure of FIG. 1C.

FIG. 1E is a cross-sectional view of a portion of a SAW resonatorstructure in accordance with a representative embodiment.

FIG. 1F is a cross-sectional view of a portion of the SAW resonatorstructure of FIG. 1C.

FIG. 2 is a flow-chart of a method of fabricating a SAW resonatorstructure according to a representative embodiment.

FIG. 3 is a simplified schematic block diagram of a filter comprising aSAW resonator structure according to a representative embodiment.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, representative embodiments disclosing specific detailsare set forth in order to provide a thorough understanding of thepresent teachings. However, it will be apparent to one having ordinaryskill in the art having had the benefit of the present disclosure thatother embodiments according to the present teachings that depart fromthe specific details disclosed herein remain within the scope of theappended claims. Moreover, descriptions of well-known apparatuses andmethods may be omitted so as to not obscure the description of therepresentative embodiments. Such methods and apparatuses are clearlywithin the scope of the present teachings.

It is to be understood that the terminology used herein is for purposesof describing particular embodiments only, and is not intended to belimiting. Any defined terms are in addition to the technical andscientific meanings of the defined terms as commonly understood andaccepted in the technical field of the present teachings.

As used in the specification and appended claims, the terms ‘a’, ‘an’and ‘the’ include both singular and plural referents, unless the contextclearly dictates otherwise. Thus, for example, ‘a device’ includes onedevice and plural devices.

As used in the specification and appended claims, and in addition totheir ordinary meanings, the terms ‘substantial’ or ‘substantially’ meanto with acceptable limits or degree. For example, ‘substantiallycancelled’ means that one skilled in the art would consider thecancellation to be acceptable.

As used in the specification and the appended claims and in addition toits ordinary meaning, the term ‘approximately’ means to within anacceptable limit or amount to one having ordinary skill in the art. Forexample, ‘approximately the same’ means that one of ordinary skill inthe art would consider the items being compared to be the same.

Relative terms, such as “above,” “below,” “top,” “bottom,” “upper” and“lower” may be used to describe the various elements' relationships toone another, as illustrated in the accompanying drawings. These relativeterms are intended to encompass different orientations of the deviceand/or elements in addition to the orientation depicted in the drawings.For example, if the device were inverted with respect to the view in thedrawings, an element described as “above” another element, for example,would now be “below” that element. Similarly, if the device were rotatedby 90° with respect to the view in the drawings, an element described“above” or “below” another element would now be “adjacent” to the otherelement; where “adjacent” means either abutting the other element, orhaving one or more layers, materials, structures, etc., between theelements.

In accordance with a representative embodiment, a SAW resonatorstructure comprises a substrate having a first surface and a secondsurface. The first surface of the substrate has a plurality of features.A piezoelectric layer is disposed over the substrate. The piezoelectriclayer has a first surface and a second surface. A plurality ofelectrodes is disposed over the first surface of the piezoelectriclayer, and the plurality of electrodes is configured to generate surfaceacoustic waves in the piezoelectric layer. The SAW resonator structurealso comprises a layer disposed between the first surface of thesubstrate and the second surface of the piezoelectric layer, the firstsurface of the layer having a smoothness sufficient to foster atomicbonding between the first surface of the layer and the second surface ofthe piezoelectric layer, wherein the plurality of features reflectacoustic waves back into the piezoelectric layer.

FIG. 1A is a top view of a SAW resonator structure 100 according to arepresentative embodiment. Notably, the SAW resonator structure 100 isintended to be merely illustrative of the type of device that canbenefit from the present teachings. Other types of SAW resonators,including, but not limited to a dual mode SAW (DMS) resonators, andstructures therefore, are contemplated by the present teachings. The SAWresonator structure 100 of the present teachings is contemplated for avariety of applications. By way of example, and as described inconnection with FIG. 3, a plurality of SAW resonator structures 100 canbe connected in a series/shunt arrangement to provide a ladder filter.

The SAW resonator structure 100 comprises a piezoelectric layer 103disposed over a substrate (not shown in FIG. 1A). In accordance withrepresentative embodiments, the piezoelectric layer 103 comprises on of:lithium niobate (LiNbO₃), which is commonly abbreviated LN; or lithiumtantalate (LiTaO₃), which is commonly abbreviated LT.

The SAW resonator structure 100 comprises an active region 101, whichcomprises a plurality of interdigitated electrodes 102 disposed over apiezoelectric layer 103, with acoustic reflectors 104 situated on eitherend of the active region 101. In the presently described representativeembodiment, electrical connections are made to the SAW resonatorstructure 100 using the busbar structures 105.

As is known, the pitch of the resonator electrodes determines theresonance conditions, and therefore the operating frequency of the SAWresonator structure 100. Specifically, the interdigitated electrodes arearranged with a certain pitch between them, and a surface wave isexcited most strongly when its wavelength λ is the same as the pitch ofthe electrodes. The equation f₀=v/λ describes the relation between theresonance frequency (f₀), which is generally the operating frequency ofthe SAW resonator structure 100, and the propagation velocity (v) of asurface wave. These SAW waves comprise Rayleigh or Leaky waves, as isknown to one of ordinary skill in the art, and form the basis offunction of the SAW resonator structure 100.

Generally, there is a desired fundamental mode, which is typically aLeaky mode, for the SAW resonator structure 100. By way of example, ifthe piezoelectric layer 103 is a 42° rotated LT, the shear horizontalmode, having a displacement in the plane of the interdigitatedelectrodes 102 (the x-y plane of the coordinate system of FIG. 1A). Thedisplacement of this fundamental mode is substantially restricted tonear the upper surface (first surface 110 as depicted in FIG. 1C) of thepiezoelectric layer 103. It is emphasized that the 42° rotated LTpiezoelectric layer 103, and the shear horizontal mode are merelyillustrative of the piezoelectric layer 103 and desired fundamentalmode, and other materials and desired fundamental modes arecontemplated.

However, other undesired modes, which are often referred to as spuriousmodes, are established. Turning to FIG. 1B, a graph of admittance versusfrequency is depicted for the illustrative 42° rotated LT piezoelectriclayer 103. The desired fundamental mode, the shear horizontal mode 106,is substantially restricted to the upper surface of the piezoelectriclayer 103, and has a frequency at series resonance (F_(s)). However, anumber spurious modes 107, having frequencies greater than the frequencyat parallel resonance (F_(p)), can exist in the piezoelectric layer 103.As described more fully below, these spurious modes 107 are created byacoustic waves generated in the piezoelectric layer 103 that establishstanding waves of various kinds of modes (with different modal shapesand frequencies). More specifically, these spurious modes 107 arecreated by reflections at the interface of the piezoelectric layer 103and the layer (see FIG. 1C) between the piezoelectric layer 103 and thesubstrate (see FIG. 1C) of the SAW resonator structure 100.

The spurious modes can deleteriously impact the performance of SAWresonators, and devices (e.g., filters) that include SAW resonators, ifnot mitigated. Most notably, if a first filter is comprised of one ormore SAW resonators, and is connected to a second filter having apassband that overlaps the frequency of the spurious modes, a sharpreduction in the quality (Q) of the second filter will occur. Thespurious modes are observed on a so-called Q-circle of a Smith Chart ofthe S₁₁ parameter. These sharp reductions in Q-factor are known as“rattles,” and are strongest in the southeast quadrant of the Q-circle.Beneficially, significant mitigation of the adverse impact of thesespurious modes is realized by the various aspects of the presentteachings as described below.

FIG. 1C is a cross-sectional view of the SAW resonator structure 100depicted in FIG. 1A along the lines 1B-1B. The SAW resonator structure100 comprises a substrate 108 disposed beneath the piezoelectric layer103, and a layer 109 disposed between the substrate 108 and thepiezoelectric layer 103.

As noted above, the piezoelectric layer 103 illustratively comprises oneof LN or LT. Generally, in the representative embodiments describedbelow, the piezoelectric layer 103 is a wafer that is previouslyfabricated, and that is adhered to the layer 109 by atomic bonding asdescribed more fully below.

The materials selected for the piezoelectric layer 103 can be dividedinto two types: one which has been used for a long time and with a highdegree of freedom in design is used for Rayleigh wave substrates; theother, with less freedom and limited in design, is for Leaky wavesubstrates with low loss characteristics and easily reaches the higherfrequencies by high acoustic velocity, and are mainly used for mobilecommunications. LN and LT materials are often used for broadbandfilters, and according to the filter specifications, the manufacturingmaterials and cutting angles differ. Filters for applications thatrequire comparatively low loss mainly generally require Leaky wavematerials, while Rayleigh wave materials are predominately used forcommunication equipment that requires low ripple and low group delaycharacteristics. Among Rayleigh wave materials, ST-cut crystal has thebest temperature characteristics as a piezoelectric material.

In accordance with a representative embodiment, the substrate 108comprises crystalline silicon, which may be polycrystalline ormonocrystalline, having thickness of approximately 100.0 μm toapproximately 800.0 μm. Other polycrystalline or monocrystallinematerials besides silicon are contemplated for use as the substrate 108of the SAW resonator structure 100. By way of example, these materialsinclude, but are not limited to, glass, single crystal aluminum oxide(A1 ₂O₃) (sometimes referred to as “sapphire”), and polycrystalline A1₂O₃, to name a few. In certain representative embodiments, in order toimprove the performance of a filter comprising SAW resonatorstructure(s) 100, the substrate 108 may comprise a comparativelyhigh-resistivity material. Illustratively, the substrate 108 maycomprise single crystal silicon that is doped to a comparatively highresistivity.

The layer 109 is illustratively an oxide material, such as silicondioxide (SiO₂), or phosphosilicate glass (PSG), borosilicate glass(BSG), a thermally grown oxide, or other material amenable to polishingto a high degree of smoothness, as described more fully below. The layer109 is deposited by a known method, such as chemical vapor deposition(CVD) or plasma enhanced chemical vapor deposition (PECVD), or may bethermally grown. As described more fully below, the layer 109 ispolished to a thickness in the range of approximately 0.05 μm toapproximately 6.0 μm.

The piezoelectric layer 103 has a first surface 110, and a secondsurface 111, which opposes the first surface 110. The second surface 111has a plurality of features 116 there-across. As noted above, undesiredspurious modes are launched in the piezoelectric layer 103, andpropagate down to the second surface 111. As described more fully belowin connection with portion 118 in FIG. 1D, the plurality of features 116reflect undesired spurious modes at various angles and over variousdistances to destructively interfere with the undesired spurious wavesin the piezoelectric layer 103, and possibly enable a portion of thesewaves to be beneficially converted into desired SAW waves. Again asdescribed more fully below, the reflections provided by the plurality offeatures 116 foster a reduction in the degree of spurious modes (i.e.,standing waves), which are created by the reflection of acoustic wavesat the interface of the second surface 111 of the piezoelectric layer103 and the first surface 112 of layer 109. Ultimately, the reflectionsprovided by the plurality of features 116 serve to improve theperformance of devices (e.g., filters) that comprise a plurality of SAWresonator structures 100.

The substrate 108 has a first surface 114 and a second surface 115opposing the first surface 114. In representative embodiments, thesubstrate 108 undergoes a chemical-mechanical polish (CMP) sequence toprepare the first surface to bond to the layer 109, as described below.

Layer 109 has a first surface 112 and a second surface 113. As notedabove, and as described more fully below in connection with thedescription of portion 117 in FIG. 1F, the second surface 113 of layer109 is polished, such as by chemical-mechanical polishing in order toobtain a “mirror” like finish with a comparatively low root-mean-square(RMS) variation of height. This low RMS variation of heightsignificantly improves the contact area between the second surface 113of the layer 109 and the first surface 114 of the substrate 108 toimprove the atomic bonding between the first surface 114 and the secondsurface 113. As is known, the bond strength realized by atomic bondingis directly proportional to the contact area between two surfaces. Assuch, improving the flatness/smoothness of the second surface 113fosters an increase in the contact area, thereby improving the bond ofthe layer 109 to the substrate 108. As used herein, the term atomicallysmooth means sufficiently smooth to provide sufficient contact area toprovide a sufficiently strong bond strength between the layer 109 andthe substrate 108, at the interface of their second and first surfaces113, 114, respectively.

FIG. 1D is a cross-sectional view of a portion 118 SAW resonatorstructure 100 according to a representative embodiment. Portion 118 isdepicted in FIG. 1C in magnified view to illustrate various aspects andfunctions of the plurality of features 116 of piezoelectric layer 103along the interface of the first surface 112 of the layer 109 and thesecond surface 111 of the piezoelectric layer 103.

The shape, dimensions and spacing of the features 116 depends on theirsource (e.g., method of fabrication). In certain representativeembodiments, the features 116 are provided by an unpolished wafercomprising the piezoelectric layer 103. In other representativeembodiments, the plurality of features 116 are fabricated on thepiezoelectric layer 103 using a known etching technique, and may havesides 121 with a slant that foster diffusive reflection of spuriousmodes.

Notably, some of the plurality of features 116 may have comparatively“flat” bottoms 122. The features 116 also have a height 123 that may besubstantially the same across the width of the interface between thesubstrate 108 and the layer 109. Additionally, the width (x-dimension inthe coordinate system of FIG. 1C) of the features 116 may be the same,or may be different. Illustratively, the width of the features is on theorder of the desired fundamental mode of the SAW resonator structure100.

Alternatively, and again depending on the source of the features 116(i.e., unpolished wafer or the method of fabrication), the height 123 ofthe features 116 may not be the same. Rather, by having the height 123of the features 116 not the same, a reduction in the incidence of morethan one of the spurious modes can be realized.

The representative method described presently for forming features 116are merely illustrative. Alternative methods, and thus alternative sizesand shapes of the features 116 are contemplated, and some are describedbelow. Notably, regardless of the source of the features 116, which canbe because of the unpolished wafer comprising the piezoelectric layer103, or by a known etching technique, the plurality of features 116 isbeneficially not arranged in a repetitive pattern, and thus arenon-periodic. Rather, the plurality of features 116 are typicallyrandomly located on the substrate 108, in order to avoid establishingconditions that would support standing waves (i.e., resonanceconditions) in the piezoelectric layer 103, and thereby reduce theincidence of spurious modes in the piezoelectric layer 103.

The piezoelectric layer 103 is illustratively single-crystal material,or other material having crystalline properties. Like the teachings ofthe parent application, the present teachings make use of the etchingproperties of the piezoelectric layer 103 to realize the variouscharacteristics of the features 116. However, and as described morefully below, while some etch selectivity to specific crystalline planeis possible by etching the piezoelectric layer 103 according to thepresent teachings, this does not occur nearly to the degree as the etchselectivity realized in the etching of the substrate (e.g., silicon)described in the parent application. As such, while the features 116 areetched from the piezoelectric layer 103 to a desired height having sides121 that are on a “slant” foster reflections at off-angles relative tothe incident direction of the acoustic waves 124, the more definedshapes (e.g., pyramids) provided by the selective etching of silicon,for example, as described in the parent application, are lesspronounced. However, and beneficially, the non-periodic nature of thefeatures 116 of the roughened surface provided at the second surface 111of the piezoelectric layer 103 does foster diffuse reflection ofspurious waves as described more fully below.

Turning again to FIG. 1D, acoustic waves 124 are transmitted downwardlyfrom the piezoelectric layer 103, having been generated by the SAWresonator structure 100. The acoustic waves 124 are incident on one ormore of the plurality of features 116, and are reflected therefrom.

As noted above in connection with the description of FIG. 1B, there aremultiple spurious modes, each having a different frequency andwavelength. In accordance with a representative embodiment, the height123 of the features 116 off the substrate 108 is approximatelyone-fourth (¼) λ of one or more of the spurious modes. Selecting theheight 123 of the features to be approximately one-fourth (¼) λ of aparticular spurious mode alters the phase of the reflected waves, andresults in destructive interference by the reflected waves, andsubstantially prevents the establishment of standing waves, and thusspurious modes.

In some embodiments, the height 123 of the features 116 is substantiallythe same, and the height 123 is selected to be approximately one-fourth(¼) λ of one (e.g., a predominant) of the spurious modes. In otherembodiments, the height 123 of the features 116 is not the same, butrather each different height is selected to be approximately equal toone-fourth (¼) λ of one of the multiple spurious modes (e.g., thespurious modes 107 depicted in FIG. 1B). By selecting this one height ormultiple heights, the phase of the reflected waves is altered, andresults in destructive interference by the reflected waves, therebysubstantially preventing the establishment of standing waves of multiplefrequencies, thus preventing the establishment of multiple spuriousmodes.

By way of example, if the spurious modes have a frequency of 700 MHz,the wavelength λ is approximately 6.0 μm. As such, the height 123 wouldbe approximately 1.5 μm. By contrast, if the spurious modes have afrequency of 4200 MHz, the λ is approximately 1.0 μm. In this example,the height 123 would be approximately 0.25 μm. More generally, theheight 123 is in the range of less than approximately 0.25 μm (e.g., 0.1μm) to greater than approximately 1.5 μm (e.g., 2.5 μm). As will beappreciated, the range for the height 123 depends on the frequency ofthe fundamental mode.

The non-periodic orientation of the plurality of features 116, thegenerally, angled surfaces (e.g., side 121) provided by the plurality offeatures 116, and providing the height 123 of the features 116 to be inthe noted range relative to the wavelength of the propagating spuriousmodes combine to alter the phase of the acoustic waves 124 incident onthe various features. Beneficially, these factors in combination resultin comparatively diffuse reflection of the acoustic wave back throughthe piezoelectric layer 103. This comparatively diffuse reflection ofthe acoustic waves from the features 116 will generally not fosterconstructive interference, and the establishment of resonanceconditions. Accordingly, the plurality of features 116 generally preventthe above-noted parasitic acoustic standing waves (i.e., spurious modes)from being established from the acoustic waves 124 generated in thepiezoelectric layer 103, which travel down and into the substrate 108.

One measure of the impact of the parasitic spurious modes on theperformance of a device (e.g., filter) comprising a SAW resonator is thequality (Q) factor. For example, the parasitic spurious modes couple atthe interfaces of the piezoelectric layer 103 and remove energyavailable for the desired SAW modes and thereby reduce the Q-factor ofthe resonator device. As is known, the Q-circle of Smith Chart has avalue of unity along its circumferences. The degree of energy loss (andtherefore reduction in Q) is depicted with the reduction of the S₁₁parameter off the unit circle. Notably, as a result of parasiticspurious modes and other acoustic losses, sharp reductions in Q of knowndevices can be observed on a so-called Q-circle of a Smith Chart of theS₁₁ parameter. These sharp reductions in Q-factor are known as “rattles”or “,” and are strongest in the southeast quadrant of the Q-circle.Beneficially, because of the diffuse reflections, and attendant phasemismatch of the reflected acoustic waves 124 realized by the pluralityof features 116, compared to such known devices, a filter comprising SAWresonator structure 100 of representative embodiments of the presentteachings, show lesser magnitudes of the “rattles” or “,” and a somewhat“spreading” of the reduced “rattles” is experienced.

As noted above, the plurality of features 116 may be formed by etchingthe piezoelectric layer 103. In one embodiment, the piezoelectric layer103 is substantially monocrystalline LT or LN that is etched using knownwet or dry etching techniques. By way of example, an etch-resistant maskis selectively provided over the piezoelectric layer 103 and a wet etchis carried out, achieving some degree of selectivity. Illustratively, anetch resistant mask is patterned, and etching is effected by the use ofan anisotropic etchant such as hydrofluoric acid (HF), or a mixture ofHF and HNO₃. As noted above, the etching sequence used to form thefeatures 116 in the piezoelectric layer 103 will result in the features116 being less defined than, for example, the pyramids formed in thesubstrate as described in the parent application, but nonethelessdefined enough to reveal sides 121 with slants and flat bottoms 122 asdepicted in FIGS. 1D and 1E. Moreover, the depth of the etch, andtherefore the height 123 of the features 116, is, of course, controlledby the duration of the etch. Therefore, the magnitude of the height 123is beneficially well-controlled.

In one representative method, the etch mask is patterned to provide“dots” in rather random locations over the second surface 111 of thepiezoelectric layer 103. Illustratively, the “dots” are resistant to adry-etchant used in a dry-etching technique, such as aninductively-coupled plasma etching technique, known to one of ordinaryskill in the art. After etching, these “dots” are removed, and show theflat bottoms 122 of certain ones of the plurality of features 116. Thespacing of the “dots” and the duration of the etch determines the depthof each etch, and therefore, the height 123 of the resultant features116.

Again, the use of monocrystalline LT or LN for the piezoelectric layer103 is merely illustrative, and other materials can be processed toprovide the plurality of features 116 described above.

In other representative embodiments, the plurality of features 116 hasrandom spacing, or random orientation, or random heights, or acombination thereof. The random aspect of the orientation of thefeatures 116 can result from a fabrication step, or from the unpolishedwafer comprising the piezoelectric layer 103. As can be appreciated,such random spacings, orientations and heights, alone or in combinationcan foster comparatively diffuse reflection of the acoustic waves 124incident thereon. This diffuse reflection, in turn, alters the phase ofthe acoustic waves, and serves to reduce the propensity of standingwaves (and thus spurious modes) from being established.

The random spacing, orientation, and heights of the plurality offeatures can be effected by a number of methods. For example, theplurality of features 116 may be provided by simply using an unpolishedwafer comprising the piezoelectric layer 103. Alternatively, the secondsurface 115 of the substrate 108 could be rough polished by CMP, forexample, or grinded, or otherwise etched in a random manner. While theplurality of features 116 of such an embodiment would likely not havethe height relative to the wavelength of the spurious modes, the randomnature of such an unpolished surface would likely provide a usefuldegree of diffusive reflection to avoid the establishment of a resonantcondition for the spurious modes.

FIG. 1E is a cross-sectional view of a portion of a SAW resonatorstructure according to a representative embodiment. Many aspects anddetails of the various features and their methods of fabricationdescribed in connection with the representative embodiments of FIG. 1Eare common to those described above in connection with therepresentative embodiments of FIGS. 1A-1D. Such common aspects anddetails are often not repeated in order to avoid obscuring thedescription of the present representative embodiments.

Notably, the portion depicted in FIG. 1E is somewhat similar to portion118 depicted in FIG. 1D, however differs in the depth of the polishingstep used to provide first surface 112. Specifically, rather thanterminating the polishing of the layer 109 at a height significantlyabove the features 116, the polishing step continues and in placesreveals the features 116. This polishing step thus provides, in places,comparatively “flat” bottoms 122. By contrast, in other places, thefeatures 116 are not altered by the polishing.

Like the plurality of features 116 depicted in FIG. 1D, the features 116of the representative embodiments of FIG. 1E are formed by processingpiezoelectric layer 103 by one of a number of methods, such as describedabove, or by providing an unpolished wafer comprising the piezoelectriclayer 103. As noted above, using a known etching method for theirformation, the resultant features 116 may have sides 121 that are on a“slant,” and foster reflections at off-angles relative to the incidentdirection of the acoustic waves 124. Similarly, like the plurality offeatures 116 of FIG. 1D, the height 123 of the plurality of features 116of the representative embodiments of FIG. 1E is approximately one-fourth(¼) λ of one or more of the spurious modes. Selecting the height 123 ofthe features 116 to be approximately one-fourth (¼) λ of a particularspurious mode alters the phase of the reflected waves, and results indestructive interference by the reflected waves, and substantiallyprevents the establishment of standing waves, and thus spurious modes.

In some embodiments, the height 123 of the features 116 is substantiallythe same, and thus the height 123 is selected to be approximatelyone-fourth (¼) λ of one (e.g., a predominant) spurious mode. In otherembodiments, the height 123 of the features 116 is not the same, butrather each different height is selected to be approximately equal toone-fourth (¼) λ of one of the multiple spurious modes (e.g., one of thespurious modes 107 depicted in FIG. 1B). By selecting such multipleheights, the phase of the reflected waves is altered, and results indestructive interference by the reflected waves, thereby substantiallypreventing the establishment of standing waves of multiple frequencies,thus preventing the establishment of multiple spurious modes.

In other representative embodiments, the plurality of features 116 haverandom spacing, or random orientation, or random heights, or acombination thereof. As can be appreciated, such random spacings,orientations and heights, alone or in combination can fostercomparatively diffuse reflection of the acoustic waves 124 incidentthereon. This diffuse reflection, in turn, alters the phase of theacoustic waves, and serves to reduce the propensity of standing waves(and thus spurious modes) from being established.

The random spacing, orientation, and heights of the plurality offeatures can be effected by a number of methods. For example, theplurality of features 116 may be provided by simply using an unpolishedwafer for the substrate 108. Alternatively, the second surface 115 ofthe substrate 108 could be rough polished by CMP, for example. While theplurality of features 116 of such an embodiment would likely not havethe height relative to the wavelength of the spurious modes, the randomnature of such an unpolished surface would likely provide a usefuldegree of diffusive reflection to avoid the establishment of a resonantcondition for the spurious modes.

FIG. 1F is a cross-sectional view of a portion 117 SAW resonatorstructure 100 according to a representative embodiment. Portion 117 isdepicted in FIG. 1F in magnified view to illustrate various aspects andfunctions of the layer 109 along the interface of the layer 109 and thesubstrate 108.

In a representative embodiment, layer 109 is PSG, which is depositedover the piezoelectric layer 103. Illustratively, the PSG is depositedat a temperature of approximately 450° C., using silane and P₂O₅ sourcesto form a soft glass like material which is approximately 8%phosphorous. This low temperature process is well known to those skilledin the art, and hence, will not be discussed in detail here.

Unfortunately, at the atomic level the surface of such deposited filmsare atomically very rough. However, the second surface 113 of layer 109(e.g., PSG) can be polished by a known method to provide an atomicallysmooth surface. The surface of the layer 109 is first planarized bypolishing with a slurry, using a known CMP method. The remaining PSG canthen be polished using a more refined slurry. Alternatively, a singlemore refined slurry can be used for both polishing steps if theadditional polishing time is not objectionable. As noted above, the goalis to create a “mirror” like finish that is atomically smooth in orderto foster strong atomic bonding between the layer 109 and the substrate108, at the interface of their second and first surfaces 113, 114respectively. Further details of the polishing sequence can be found,for example, in U.S. Pat. No. 6,060,818 and U.S. Patent ApplicationPublication 20050088257, to Ruby, et al. The entire disclosures of U.S.Pat. No. 6,060,818, and U.S. Patent Application Publication 20050088257are specifically incorporated herein by reference.

FIG. 1F depicts four “humps” 125 in the layer after the completion ofthe cleaning of the wafer to remove remnants of the CMP process, asdescribed, for example, in above-incorporated U.S. Pat. No. 6,060,818and U.S. Patent Application Publication 20050088257, to Ruby, et. al.The “humps” depict variation in the second surface 113 of the layer 109.The first hump has a first height, H₁, the second hump has a secondheight, H₂, the third hump has a third height, H₃, and the fourth humphas a fourth height, H₄. For the purposes of illustration, only fourhumps are shown. The root mean squared (RMS) variation in the height ofthe second surface 113 of the layer 109 comprised of the four humpsdepicted is less than approximately 0.5 μm. As noted above, the termatomically smooth herein means sufficiently smooth to provide sufficientcontact area to provide a sufficiently strong bond strength between thelayer 109 and the substrate 108, at the interface of their second andfirst surfaces 113, 114, respectively. Such an atomically smooth surfacecan be realized by providing the second surface 113 of layer 109 havingan RMS variation in height of in the range of approximately 0.1 Å toapproximately 10.0 Å; although beneficially, the RMS variation in heightis less than approximately 5.0 Å.

As noted above, the forming of an atomically smooth second surface 113provides an increased contact area at the interface of the second andfirst surfaces 113, 114, respectively, of the layer 109 and thesubstrate 108. This increased contact area, in turn, fosters acomparatively strong atomic bond between the layer 109 and the substrate108. Among other benefits, the strong atomic bond between the layer 109and the substrate 108 reduces separation or delamination of the layer109 and the substrate 108, thereby increasing the reliability of devicescomprising the SAW resonator structure 100 over time.

FIG. 2 is a flow-chart of a method 200 of fabricating a SAW resonatorstructure according to a representative embodiment. Many aspects anddetails of the method, including illustrative materials, processes, anddimensions are common to those described above. These aspects anddetails may not be repeated to avoid obscuring the presently describedrepresentative embodiment.

At 201, the method begins with providing a piezoelectric layer a firstsurface and a second surface. As noted above, among other materials, thepiezoelectric layer (e.g., piezoelectric layer 103) may bemonocrystalline LT or LN.

At 202, a plurality of features (e.g., plurality of features 116) areetched into the piezoelectric layer 103.

At 203, a layer is provided over a second surface of the piezoelectriclayer, which comprises the plurality of features. As noted above, thelayer (e.g., layer 109) may be an oxide (glass), such as SiO₂ or PSG.

At 204, a second surface (e.g., 113) of the layer is polished to providea comparatively smooth second surface. As noted above a CMP method maybe used to provide a first surface that is atomically smooth to foster astrong atomic bond between the layer (e.g. layer 109) and the substrate(e.g. substrate 108).

At 205, the method comprises atomically bonding the layer to the firstsurface of the substrate. Notably, this bonding comprises contacting thesubstrate (e.g., substrate 108) with the layer (e.g., layer 109) thathas been polished to an atomically smooth degree.

As noted above, when connected in a selected topology, a plurality ofSAW resonators can function as an electrical filter. FIG. 3 shows asimplified schematic block diagram of an electrical filter 300 inaccordance with a representative embodiment. The electrical filter 300comprises series SAW resonators 301 and shunt SAW resonators 302. Theseries SAW resonators 301 and shunt SAW resonators 302 may each compriseSAW resonator structures 100 described in connection with therepresentative embodiments of FIGS. 1A˜2. As can be appreciated, the SAWresonator structures (e.g., a plurality of SAW resonator structures 100)that comprise the electrical filter 300 may be provided over a commonsubstrate (e.g., substrate 108), or may be a number of individual SAWresonator structures (e.g., SAW resonator structures 100) disposed overmore than one substrate (e.g., more than one substrate 108). Theelectrical filter 300 is commonly referred to as a ladder filter, andmay be used for example in duplexer applications. It is emphasized thatthe topology of the electrical filter 300 is merely illustrative andother topologies are contemplated. Moreover, the acoustic resonators ofthe representative embodiments are contemplated in a variety ofapplications including, but not limited to duplexers.

The various components, materials, structures and parameters areincluded by way of illustration and example only and not in any limitingsense. In view of this disclosure, those skilled in the art canimplement the present teachings in determining their own applicationsand needed components, materials, structures and equipment to implementthese applications, while remaining within the scope of the appendedclaims.

1. A surface acoustic wave (SAW) resonator structure, comprising:substrate having a first surface and a second surface; a piezoelectriclayer disposed over the substrate, the piezoelectric layer having afirst surface, and a second surface comprising a plurality of features;a plurality of electrodes disposed over the first surface of thepiezoelectric layer, the plurality of electrodes configured to generatesurface acoustic waves in the piezoelectric layer; and a layer disposedbetween the first surface of the substrate and the second surface of thepiezoelectric layer, the second surface of the layer having a smoothnesssufficient to foster atomic bonding between the second surface of thelayer and the first surface of the substrate, wherein the plurality offeatures reflect acoustic waves and reduce the incidence of spuriousmodes in the piezoelectric layer.
 2. A SAW resonator structure asclaimed in claim 1, wherein the reflected acoustic waves destructivelyinterfere with acoustic waves in the piezoelectric layer.
 3. A SAWresonator structure as claimed in claim 1, wherein at least some of theplurality of features have substantially slanted sides.
 4. A SAWresonator structure as claimed in claim 3, wherein at least some of theplurality of features are substantially not in a regular pattern.
 5. ASAW resonator structure as claimed in claim 3, wherein the at least someof the plurality of features have a height of approximately one-fourthof a wavelength (¼ λ) of a spurious mode.
 6. A SAW resonator structureas claimed in claim 3, wherein the at least some of the plurality offeatures have a height in the range of approximately 0.25 μm toapproximately 1.5 μm.
 7. A SAW resonator structure as claimed in claim3, wherein the at least some of the plurality of features have a heightin the range of approximately 0.1 μm to approximately 2.50 μm.
 8. A SAWresonator structure as claimed in claim 3, wherein the at least some ofthe plurality of features have a plurality of heights, and each of thepluralities of heights is approximately a height in the range ofapproximately one-fourth of a wavelength (¼ λ) of one of the pluralityof spurious modes.
 9. A SAW resonator structure as claimed in claim 1,wherein the layer comprises an oxide material.
 10. A SAW resonatorstructure as claimed in claim 9, wherein the oxide material comprisessilicon dioxide (SiO₂).
 11. A SAW resonator structure as claimed inclaim 9, wherein the second surface of the layer has a root-mean-square(RMS) variation in height of approximately 0.1 Å to approximately 10.0Å.
 12. A surface acoustic wave (SAW) filter comprising a plurality ofSAW resonator structures, one or more of the plurality of SAW resonatorstructures comprising: substrate having a first surface and a secondsurface; a piezoelectric layer disposed over the substrate, thepiezoelectric layer having a first surface, and a second surfacecomprising a plurality of features; a plurality of electrodes disposedover the first surface of the piezoelectric layer, the plurality ofelectrodes configured to generate surface acoustic waves in thepiezoelectric layer; and a layer disposed between the first surface ofthe substrate and the second surface of the piezoelectric layer, thesecond surface of the layer having a smoothness sufficient to fosteratomic bonding between the second surface of the layer and the firstsurface of the piezoelectric layer, wherein the plurality of featuresreflect acoustic waves and reduce the incidence of spurious modes in thepiezoelectric layer.
 13. A SAW filter as claimed in claim 12, whereinthe reflected acoustic waves destructively interfere with acoustic wavesin the piezoelectric layer.
 14. A SAW filter as claimed in claim 12,wherein at least some of the plurality of features in the first surfaceof the substrate have substantially slanted sides.
 15. A SAW filter asclaimed in claim 12, wherein the plurality of features are substantiallynot in a regular pattern.
 16. A SAW filter as claimed in claim 12,wherein the at least some of the plurality of features have a height ofapproximately one-fourth of a wavelength (¼ λ) of a spurious mode.
 17. ASAW filter as claimed in claim 16, wherein the at least some of theplurality of features have a height in the range of approximately 0.25μm to approximately 1.5 μm.
 18. A SAW filter as claimed in claim 16,wherein the at least some of the plurality of features have a height inthe range of approximately 0.1 μm to approximately 2.50 μm.
 19. A SAWfilter as claimed in claim 16, wherein the at least some of theplurality of features have a plurality of heights, and each of thepluralities of heights is approximately a height in the range ofapproximately one-fourth of a wavelength (¼ λ) of one of the pluralityof spurious modes.
 20. A SAW filter as claimed in claim 12, wherein theSAW filter is a ladder filter, comprising a plurality of SAW resonatorsstructures.
 21. A SAW filter as claimed in claim 12, wherein the layercomprises an oxide material.
 22. A SAW filter as claimed in claim 21,wherein the oxide material comprises silicon dioxide (SiO₂).
 23. A SAWfilter as claimed in claim 21, wherein the second surface of the layerhas a root-mean-square (RMS) variation in height of approximately 1.0 Åto approximately 10.0 Å or less.
 24. A SAW filter as claimed in claim11, wherein two or more of the plurality of resonators are configured ina series and shunt configuration.